Designing, building or using laser based systems requires instrumentation for measuring the laser beam characteristics. It would be particularly desirable to provide an instrument for simultaneous measuring of various beam parameters, such as: spatial propagation of a Laser Beam; degree of collimation; optical power; optical power noise; beam intensity profile across the transverse area of the beam at any selected position; and beam intensity distribution across the transverse plane.
These parameters are further explained herein.
Spatial propagation of a laser beam: Following recent developments in the laser theory, pioneered by A. E. Siegman, a laser beam is comprised of higher modes. Its spatial propagation formula can be described by the formula: ##EQU1## where W.sub.z is the beam radius at the waist, located at a distance Z along the arbitrary XYZ axis system, M.sup.2 is a multi-mode beam quality parameter and k is the laser wavelength.
For a perfect Gaussian beam M.sup.2 =1. The above formula is appropriate for laser beams which are circularly symmetric about their propagation axis. A similar formula can be developed for the general case, where the beam is astigmatic and has an elliptical cross-section.
Degree of Collimation: Light of laser beams travels through space in straight lines, having an optical cone diverging from or converging off the light source.
The amount of divergence/convergence defines the degree of beam collimation, a parameter which is very important for beam characterization.
Optical power: The amount of incident light power inside the beam.
Optical power noise: Variations in power stability as a function of time.
Beam intensity profile across the transverse area of the beam, at a preselected position: Light distribution in a plane across the laser beam propagation, when scanned in one direction.
Beam intensity distribution across the transverse plane: Two dimensional mapping of beam intensity across the beam.
Much effort has been dedicated towards a standardization of beam width measurements, beam divergence (collimation) and beam propagation factor (M2), which were summarized in an ISO (International Organization for Standardization) proposal for working draft, ISO/TC172/SC9 WG1, Project 2, Document N38 Revised. The IS0 publication describes three standards for measuring beam parameters:
A Standard: Moving Slit Scan Method PA1 B Standard: Moving Knife-Edge Method PA1 C Standard: Variable Aperture Method PA1 The beam power or energy in the measurement region must be reduced below the level, where the detector saturation (or damage) occurs, but also as importantly below level where optical components are distorted by the thermal effects of the incident energy. PA1 The way for distortion to the attenuator must not cause measurable changes in the measured beam diameters immediately after the attenuation train is exposed to the full power/energy being measured. PA1 The standard requires that a laser beam be brought to the real focus, using an aberration free (aberration shall not increase the focus beam width), focusing element of focal length known within 2% at the wavelength of interest.
U.S. Pat. Nos. 5,078,491 to Johnston, 5,069,527 to Johnston et al, and 5,064,284 to Johnston et al, (the '284 patent) teach measurement of laser beam quality. These patents describe laser beam analyzers which measure laser beams as to their mode composition and other qualities. The laser beam is brought to a focus by a lens (32, 132, 532, or 632 in the '284 patent), creating a transformed beam waist a distance f away from the lens, where f is the focal length of the lens. The lateral power profile of the beam is taken at various points along the beam in the region of the transformed waist to find the M.sup.2 parameter of the beam. The beam profiles are obtained by chopping the beam with a moving knife edge or straight edge and recording the eclipse profile with a detector placed in the beam beyond the chopper.
The eclipse profile is the power of the beam, incident on the detector, as a function of time. The mode composition and parameter M.sup.2 can be obtained, by simple conventional tomography, from this profile. The Johnston system chops the beam with a rotating drum or hub (34, 134, 534, or 634 in the '284 patent) whose cylindrical side contains an aperture (36, 136, etc. in the '284 patent). The aperture is bordered by the knife edges which cut the beam as the hub spins on its axis. Across the hub from the aperture is an opening or window (38, etc.) which is larger than the aperture to allow the beam to shine through the hub 34 onto the detector 30.
One of the applications of the Johnston invention is to measure the M.sup.2 parameter at various places along a beam line in order to find out which optical components are misaligned or faulty; such components lower the M.sup.2 value. To do this, the device must be installed in the beam line between the component under test and the next component down the beam line. However, the Johnston device is rather long, and thus may be difficult to insert in between optical components which are quite inconvenient to move once they are aligned.
There are two main reasons for the length of the Johnston device: first, in operation the lens 32 must be moved a distance along the beam line at least as great as the diameter D of the hub 34 (see the '284 patent at column 24, lines 42-47) and generally equal to half of the focal length f of the lens 32 (see the '284 patent at column 28, line 30); second, the detector 30 is placed beyond the hub 34, so that the Johnston apparatus must accommodate the focal length of the lens, the diameter D of the hub, and the length of the detector 30 with its housing as well.
The focal length f of the lens is thus the major component of the device length. The length f should be short for measuring the parameter M.sup.2 in between close-set components of an optical train, and for reducing the mechanical requirements of the mounting needed to hold the lens rigidly relative to the hub and detector. On the other hand, the focal length f should be long to reduce lens aberrations. (There is a relationship between the hub diameter D and the lens focal length f, as taught by Johnston et al, at column 29, lines 20-25 and Table 1 of the '284 patent; they use a 20-cm focal-length lens.)
Of the various lens aberrations, spherical aberration is the most important. (Chromatic aberration is substantially unimportant for laser beams because they are highly monochromatic.) Spherical aberration varies as the inverse cube of f, so that doubling f reduces the spherical aberration by eight times. With a long focal length f, a simple and inexpensive plano-convex lens will have low enough spherical aberration to be used in a laser beam analyzer of the Johnston type.
For a given amount of spherical aberration (i.e., for a given optical quality), the cost of a lens goes up rapidly as f decreases. Conversely, for a fixed lens price the aberration increases as f shrinks. Thus, a physically short device of the Johnston type must use an expensive lens if the beam image is to be good enough for measuring the beam parameters near the beam waist.
If the device is to be used with various lasers having differing wavelengths, then the lens should also be adjustable, as taught by Johnson et al in the '284 patent, to optimize the performance of the device. In FIG. 27 and at column 38, line 43 et seq of the '284 patent, the patentees present a complex, multi-element telephoto design for the lens 32.
An alternative lens design, not taught by Johnston et al, is to use interchangeable lenses of simple plano-convex design. However, such lenses are limited to a focal ratio or f-ratio of f/16 or greater. ("f/16" means that the focal length f is 16 times the lens diameter.) Since the lens diameter must be about 1.5 times the beam width, the focal length f must be quite long.
The '284 patent teaches measurement of astigmatic beams in column 11, line 67 et seq. At column 17, line 17 it states that two knife edges at 90.degree. to each other (each at 45.degree. to the hub plane) "should" be used. This arrangement requires, if the angle of astigmatism is to be measured, that the entire device be made rotatable about the beam axis by means of a collet, as stated at column 17, line 64 and discussed at column 26, line 25 et seq. One embodiment is discussed at column 27, line 61.
The provision of a collet or other means for rotation about the beam axis complicates the mechanical structure and the measurement process. The main disadvantages of this approach are the high price of the moving lens system and the fact that not all laser or generic light beam parameters can be measured directly.